Acyclic or Long-Bond Intermediate in the Electron-Transfer-Catalyzed Dimerization of 4-Methoxystyrene

The electron-transfer-catalyzed dimerization of 4-methoxystyrene has long been a prototypical reaction for the study of radical cation reactivity. The different possible pathways were explored at the B3LYP/6-31G* level of theory. Both [2 + 2] and [4 + 2] cycloadditions proceed via a stepwise pathway, diverging at an acyclic intermediate and interconnected by a vinylcyclobutane-type rearrangement. The experimentally observed stereoselectivity of the cycloaddition was traced to relatively high barriers for isomerization, while the previously described “long-bond” intermediate could not be located at the higher level of theory. CPCM calculations show that the highly exothermic [4 + 2] pathway becomes kinetically more favorable in condensed phase. Time-dependent density functional theory calculations indicate that the different possible intermediates have very similar absorption spectra, making the unambiguous assignment of the experimentally observed transient absorption of 500 nm to a given species difficult.